Recently, devices used in the higher frequency ranges than conventionally used, for example, electric car motors, gas microturbines, and high-frequency reactors, have been increasingly used, and there has been a demand for electrical steel sheets having excellent magnetic properties in the high frequency range. These devices are used in a high frequency range of several hundred hertz to several tens of kilohertz.
Conventionally, in such applications, Fe—Si based non-oriented electrical steel sheets in which iron loss is improved (i.e., iron loss is decreased) by adding Si to steel have been used. In general, a non-oriented electrical steel sheet is cold-rolled so as to have a desired thickness, and is then recrystallized by final annealing to attain desired electromagnetic properties. However, in the conventional Fe—Si based non-oriented electrical steel sheet for high-frequency use, the Si content in the steel is 3.5% by mass or less, and the electrical resistance of the steel is not so high. In particular, in a high frequency range of 1 kHz or more, the iron loss is high, which is disadvantageous. Therefore, in order to meet the recent needs in society, development of new electrical steel sheets suitable for use in the high frequency range is absolutely necessary. In order to improve iron loss in the high frequency range, it is considered to be particularly effective to improve eddy current loss by increasing the electrical resistance of steel. In order to increase the electrical resistance of steel, a technique is commonly used in which the Si or Al content in the steel is increased. However, if the Si content exceeds 3.5% by mass, the steel becomes extremely hard and brittle, resulting in a deterioration in workability. Consequently, it becomes difficult to perform manufacture and working by ordinary rolling. Furthermore, in the conventional Fe—Si based steel sheet, for example, if the Si content exceeds 5.0% by mass, it becomes impossible to perform not only cold working but also warm working.
A technique for increasing the electrical resistance of steel by adding Cr, Al, etc., to steel without increasing the Si content is disclosed in Patent Document 1. However, in the technique disclosed in Patent Document 1, a frequency range of less than 1 kHz is assumed for use as in the conventional electrical steel sheet for high-frequency use, and it is not possible to obtain sufficient high-frequency magnetic properties in a frequency range of 1 kHz or more. Thus, the steel sheet disclosed in Patent Document 1 does not have a satisfactory effect as a non-oriented electrical steel sheet for high-frequency use suitable in the range of about 400 Hz to about 50 kHz required in recent years. Additionally, the Si content in Patent Document 1 does not exceed that of a typical silicon steel sheet, and rather, Patent Document 1 mainly targets a low-silicon steel sheet with a Si content of about 1.5%.
In contrast, the applicant of the present invention has found that by the addition of Cr, even in steel having a relatively high Si content, brittleness is improved, and thus both high electrical resistance and high workability are obtained. The applicant of the present invention has proposed Fe—Cr—Si based electrical steel sheets excellent in high-frequency magnetic properties with a Cr content of 1.5% to 20% by mass and a Si content of 2.5% to 10% by mass in Patent Documents 2, 3, 4, etc.                [Patent Document 1]: Japanese Unexamined Patent Application Publication No. 11-229095        [Patent Document 2]: Japanese Unexamined Patent Application Publication No. 11-343544        [Patent Document 3]: Japanese Unexamined Patent Application Publication No. 2001-262289        [Patent Document 4]: Japanese Unexamined Patent Application Publication No. 2001-279326        